L'Assassin Habite Au 21 (aka The Murderer Lives At Number 21) (1942)

A serial killer known by the name of Monsieur Durand leaves a calling card by the dead bodies of his victims. When a police detective (Pierre Fresnay) gets a tip that the killer lives at a small boarding house, he disguises himself as a clergyman in an attempt to ferret out the murderer. However, when his ditzy mistress (Suzy Delair) shows up and moves into the rooming house, she throws a monkey wrench into the proceedings. Based on the novel by Stanislas Andre Steeman and directed by Henri Georges Clouzot (DIABOLIQUE) in his feature film directorial debut. A screwball comedy about a serial killer sounds terribly politically incorrect. Even when the killer's identity is revealed, it's played for laughs. But if you can put your moral compunctions aside, you'll find a wacky but clever comedy thriller. The film drops a few red herrings along the way and they do their job because I seriously doubt you'll be able to guess the killer's identity. I thought I had but the denouement had me floored and loving that I'd been had. With Jean Tessier, Noel Roquevert, Pierre Larquey, Odette Talazac and Maximilienne.